Thermodynamics of the Association of Aqueous Calcium and Hexacyanoferrate Ions

Thermodynamic quantities have been obtained for the formation of the ion pairs Ca2+Fe(CN)(6)(4-) and Ca2+Fe(CN)(6)(3-) in dilute aqueous solution by a potentiometric method employing an ion-exchange (Orion) calcium electrode. The reversibility of the electrode was examined against calomel and silver chloride references and was further tested in a determination of the formation constant for CaSO4(aq). The mean enthalpies of ion association of calcium with both hexacyanoferrates were obtained from the temperature variation of equilibrium constants within the range 15 to 35 degrees C. The enthalpies were also independently determined calorimetrically. The results, discussed in terms of the electrostatic model, suggest that there is little penetration, if any, of hydration shells in the ion pairs.